1. Field of the Invention
The present invention relates to a technique of controlling a spread range set in a clock generator that generates a spread spectrum clock.
2. Description of the Related Art
Noise generated from any of diverse electronic devices often gives adverse electromagnetic interferences (EMI) with another electronic device. Diverse restrictions have accordingly been set with regard to the noise generated from electronic devices (hereafter referred to as ‘EMI noise’).
Manufacturers that develop and manufacture such electronic devices naturally try to minimize the EMI noise generated from their electronic devices, in order to make the electronic devices meet the restrictions of the EMI noise.
The respective units constituting a certain electronic device are generally driven and controlled in response to given clocks. The frequency spectrum of a clock typically has peaks of its amplitude at an oscillation frequency of the clock (fundamental wave: f1) and respective frequencies (f2, f3 . . . ) corresponding to its harmonies. The frequency spectrum of the EMI noise generated from the electronic device accordingly has peaks of its amplitude at the oscillation frequency (f1) of the clock and the respective frequencies (f2, f3, . . . ) corresponding to its harmonies. For reduction of the EMI noise, it is required to lower the peaks of the amplitude generated at the fundamental wave of the clock and the respective frequencies of its harmonies. One proposed technique dynamically varies the oscillation frequency of the clock output from a clock generator to spread the frequency spectrum of the clock and thereby lowers the peaks of the amplitude on the frequency spectrum generated at the fundamental wave (f1) of the clock and the respective frequencies (f2, f3 . . . ) of its harmonies. In the description hereafter, the action of varying the oscillation frequency of the clock to spread the frequency spectrum is called the ‘spread spectrum’. The variation in oscillation frequency of the clock by the spread spectrum is referred to as the ‘spread range’. The clock having the spread frequency spectrum is referred to as the ‘spread spectrum clock’.
Each manufacturer experimentally specifies in advance a spread range that meets the restrictions of the EMI noise generated from each manufactured electronic device and makes the frequency of the clock spread by the specified constant spread range.
The spread range of the clock is set in advance to a fixed value to meet the restrictions of the EMI noise as mentioned above. The setting of the spread range may, however, have adverse effects on the operations of a certain electronic device in some state of driving load of the electronic device. One typical procedure of the EMI noise measures assumes a heavy load and spreads the frequency of the clock by a designed single spread range. Even when the electronic device is actually driven in the state of a light load, the operations of the electronic device are based on the clock, which has been subjected to the spread spectrum on the assumption of a heavy load. This may adversely affect the operations of the electronic device.
For example, a projector may be used in a dark place to display a screen image with a low level of lighting luminance set in its light source unit. In such cases, a viewer who watches the screen image is relatively sensitive to a variation in luminance of the screen image. When the frequency of the clock, which is used to control the operations of the light source unit, is spread and varied, a variation in lighting luminance accompanied with the frequency variation may become prominent in some level of the spread range.